FIG. 1 is a schematic circuit diagram illustrating a capacitive touch panel system according to the prior art. As shown in FIG. 1, the capacitive touch panel system comprises driving units u1˜u6, sensing circuits s1˜s6 and a touch panel. The touch panel comprises driving electrodes d1˜d6 and receiving electrodes r1˜r6, which are not directly connected with each other. The driving electrodes d1˜d6 are connected to output terminals of respective driving units u1˜u6. The receiving electrodes r1˜r6 are connected to input terminals of respective sensing circuits s1˜s6. The equivalent capacitances Cd1˜Cd6 exist between driving electrodes d1˜d6 and the ground. The equivalent capacitances Cr1˜Cr6 exist between receiving electrodes r1˜r6 and the ground. In addition, mutual capacitances Cm11˜Cm66 exist between the driving electrodes d1˜d6 and respective receiving electrodes r1˜r6. For clarification, six driving electrodes d1˜d6 and six receiving electrodes r1˜r6 or the touch panel are shown in FIG. 1. The capacitive touch panel with more driving electrodes and more receiving electrodes may have the similar configurations, and is not redundantly described herein.
The capacitive touch panel of FIG. 1 is a multi-finger touch panel. When a conductive pointed object (e.g. a finger) touches the capacitive touch panel, the mutual capacitance value is changed. According to the change of the mutual capacitance value, a touched position is realized. Generally, once the finger of a user is placed on a touch point of the capacitive touch panel, the mutual capacitance value at the touch point is changed. Meanwhile, a driving signal is sent to the corresponding mutual capacitance. In response to the driving signal, the electric quantity stored in the mutual capacitance is correspondingly changed. The change of the electric quantity is detected by the sensing circuit. A backend circuit (not shown) connected to the sensing circuit may realize the position of the touch point according to the change of the electric quantity. Moreover, since the relationship between electric quantity (Q), voltage (V) and capacitance value (C) complies with the equation Q=C×V, the sensing circuit may also provide a voltage change to the backend circuit. The backend circuit may realize the position of the touch point according to the voltage change.
Please refer to FIG. 1 again. The six driving signals P1˜P6 will sequentially provide respective pulses to the driving electrodes d1˜d6 through the driving units u1˜u6. Since the mutual capacitances Cm11˜Cm66 are connected between the driving electrodes d1˜d6 and respective receiving electrodes r1˜r6, the coupling charge of the mutual capacitances Cm11˜Cm66 will be transmitted to the sensing circuits s1˜s6 through the receiving electrodes r1˜r6. As such, output voltages Vo1∥Vo6 are respectively generated by the sensing circuits s1˜s6.
For example, the pulse of the first driving signal P1 generated in a driving cycle T will charge the mutual capacitances Cm11˜Cm16, which are connected to the first driving electrode d1. The coupling charge of the mutual capacitances Cm11˜Cm16 will be transmitted to the sensing circuits s1˜s6 through the receiving electrodes r1˜r6. Correspondingly, output voltages Vo1˜Vo6 are respectively generated by the sensing circuits s1˜s6.
Assuming that the touch point is near the mutual capacitance Cm11, the output voltage Vo1 generated by the first sensing circuit s1 is different from the output voltages Vo2˜Vo6, which are respectively outputted from the sensing circuits s2˜s6. Whereas, assuming that two touch points are respectively near the mutual capacitances Cm11 and Cm16, the output voltages Vo1 generated by the first sensing circuit s1 and the sixth sensing circuit s6 are different from the output voltages Vo2˜Vo5, which are respectively outputted from the sensing circuits s2˜s5.
In the next driving cycles, the driving signals P2˜P6 sequentially provide pulses to the driving electrodes d1˜d6. Correspondingly, output voltages Vo1˜Vo6 are respectively generated by the sensing circuits s1˜s6.
These six driving cycles T are considered to constitute a scanning cycle τ. In other words, after the scanning cycle τ, all areas of the capacitive touch panel have been scanned once. As such, the position of the at least one touch point on the touch panel can be realized.
FIG. 2 is a schematic circuit diagram illustrating a sensing circuit of the capacitive touch panel system according to the prior art. As shown in FIG. 2, the sensing circuit s is implemented by an integrator. The sensing circuit s comprises an operation amplifier 200 and a feedback capacitor Ci. A reference voltage Vref is inputted into the positive input terminal (+) of the operation amplifier 200. Both terminals of the feedback capacitor Ci are respectively connected to the negative input terminal (−) and the output terminal Vo of the operation amplifier 200. In addition, the negative input terminal (−) of the operation amplifier 200 is also connected to the receiving electrode r. A mutual capacitance Cm is connected between the receiving electrode r and a driving electrode d. An equivalent capacitance Cr is connected between the receiving electrode r and the ground terminal GND.
During normal operation of the operation amplifier 200, the voltages inputted into the positive input terminal (+) and the negative input terminal (−) of the operation amplifier 200 are equal to the reference voltage Vref. That is, the voltage across the capacitance Cr is equal to the reference voltage Vref. In a case that the amplitude of the pulse passing through the driving electrode d is Vy, the output terminal Vo has a voltage drop ΔVo.
The voltage drop ΔVo is calculated by the equation (I): ΔVo=−Vy×Cm/Ci. Take the first driving signal P1 shown in FIG. 1 for example. In a case that no touch point is created, the mutual capacitance values of the mutual capacitances Cm11˜Cm16 are unchanged, and thus the voltage drops at the output terminals Vo1˜Vo6 of the sensing circuits s1˜s6 are identical. On the other hand, if the touch point is near the mutual capacitance Cm11, the mutual capacitance value of the mutual capacitance Cm11 is changed, and thus the voltage drop at the output terminal Vo1 of the first sensing circuit s1 is different from the voltage drops at the output terminals Vo2˜Vo6 of the sensing circuit s2˜s6. According to the voltage drops at the output terminals Vo1˜Vo6 of the sensing circuit s1˜s6, the backend circuit may realize the position of the touch point.
If the change of the mutual capacitance value of the mutual capacitance Cm at the touch point is very small, the coupling charge of the mutual capacitance Cm is slightly different from the coupling charge of other mutual capacitances. As such, the voltage drop generated by the sensing circuit corresponding to the touch point is slightly different from the voltage drops generated by other sensing circuits. In this situation, the backend circuit fails to realize the position of the touch point according to the change of the voltage drop.
FIG. 3 is a schematic circuit diagram illustrating another capacitive touch panel system according to the prior art. As shown in FIG. 3, each of the driving signals P1˜P6 generates two pulses t1 and t2 in a driving cycle T. In other words, the coupling charge of the mutual capacitances will be generated in several times. The sensing circuits s1˜s6 are designed to accumulate the coupling charge of the mutual capacitances in several times. As such, the output voltages Vo1˜Vo6 from the sensing circuits s1˜s6 are distinguishable.
As shown in FIG. 3, a scanning cycle τ includes six driving cycles T, and two pulses t1 and t2 are generated in each driving cycle T. That is, since each of the driving signals P1˜P6 generates two pulses t1 and t2 in each driving cycle T, the coupling charge of the mutual capacitances will be generated in several times. The sensing circuits s1˜s6 are designed to accumulate the coupling charge of the mutual capacitances in several times and generate a higher voltage drop for determining the position of the touch point. In other words, after the scanning cycle τ, all areas of the capacitive touch panel have been scanned once. As such, the position of the at least one touch point on the touch panel can be realized.
For clarification, two pulses t1 and t2 generated in each driving cycle T are shown in FIG. 3. It is noted that more than two pulses may be generated in each driving cycle T. As such, the sensing circuits s1˜s6 generate a higher voltage drop. The use of multiple pulses to accumulate the coupling charge of the mutual capacitances is disclosed in for example U.S. Pat. No. 6,452,514, which is entitled “Capacitive sensor and array”.